Biomaterials that can form gels in situ are useful for a variety of applications. In many cases, in situ gel-forming materials are used as injectable matrices for controlled drug delivery or injectable scaffolds for tissue engineering. (Gutowska A, Jeong B, Jasionowski M. Anat Rec 2001, 263, 342-349. Silva E A, Mooney D J. J Thromb Haemost 2007, 5, 590-8. Mahoney M J, Anseth K S. J Biomed Mater Res A 2007, 81, 269-78.) In situ gel-forming materials can also serve as adhesives to bond tissue or seal leaks (either gas or fluid) in a physiological environment.
Interest in soft tissue adhesives is growing because of the desire to replace or supplement sutures for wound closure (Glickman M, Gheissari A, Money S, Martin J, Ballard J. Arch Surg 2002, 137, 326-31; discussion 332. Pursifull N F, Morey A F. Curr Opin Urol 2007, 17, 396-401), the trends toward less invasive and cosmetic surgeries (Tissue Adhesives in Clinical Medicine; 2nd ed.; Quinn, J. V., Ed.; B C Decker: Hamilton, Ontario Canada, 2005. Tissue Glue in Cosmetic Surgery; Saltz, R.; Toriumi, D. M., Eds.; Quality Medical Publishing, Inc.: St. Louis, Mo., USA 2004), and the need for emergency hemostasis (Pusateri A E, Holcomb J B, Kheirabadi B S, Alam H B, Wade C E, Ryan K L. Journal of Trauma-Injury Infection and Critical Care 2006, 60, 674-682. Acheson E M, Kheirabadi B S, Deguzman R, Dick E J, Holcomb J B. Journal of Trauma-Injury Infection and Critical Care 2005, 59, 865-874. Kheirabadi B S, Acheson E M, Deguzman R, Sondeen J L, Ryan K L, Delgado A, Dick E J, Holcomb J B. Journal of Trauma-Injury Infection and Critical Care 2005, 59, 25-34.)
In situ gel formation can be initiated by a variety of approaches. Chemical approaches to gel formation include the initiation of polymerization either by contact, as in cyanoacrylates, or external stimuli such as photo-initiation. Also, gel formation can be achieved by chemically crosslinking pre-formed polymers using either low molecular weight crosslinkers such as glutaraldehyde or carbodiimide (Otani Y, Tabata Y, Duda Y. Ann Thorac Surg 1999, 67, 922-6. Sung H W, Huang D M, Chang W H, Huang R N, Hsu J C. J Biomed Mater Res 1999, 46, 520-30. Otani, Y.; Tabata, Y.; Ikada, Y. Biomaterials 1998, 19, 2167-73. Lim, D. W.; Nettles, D. L.; Setton, L. A.; Chilkoti, A. Biomacromolecules 2008, 9, 222-30), or activated substituents on the polymer (Iwata, H.; Matsuda, S.; Mitsuhashi, K.; Itoh, E.; Ikada, Y. Biomaterials 1998, 19, 1869-76).
In addition to chemical approaches, gel formation can be achieved through physical means using self-assembling peptides (Ellis-Behnke R G, Liang Y X, Tay D K, Kau P W, Schneider G E, Zhang S, Wu W, So K F. Nanomedicine 2006, 2, 207-15. Haines-Butterick L, Rajagopal K, Branco M, Salick D, Rughani R, Pilarz M, Lamm M S, Pochan D J, Schneider J P. Proc Natl Acad Sci USA 2007, 104, 7791-6. Ulijn R V, Smith A M. Chem Soc Rev 2008, 37, 664-75).
Finally, biological approaches to initiate gel formation have been investigated based on the crosslinking components from marine adhesives, such as mussel glue (Strausberg R L, Link R P. Trends Biotechnol 1990, 8, 53-7), or blood coagulation, as in fibrin sealants (Jackson M R. Am J Surg 2001, 182, 1S-7S. Spotnitz W D. Am J Surg 2001, 182, 8S-14S Buchta C, Hedrich H C, Macher M, Hocker P, Redl H. Biomaterials 2005, 26, 6233-41.27-30).
A variety of biomimetic approaches have also been considered for in situ gel formation. In these approaches, polymer crosslinking and gel formation are modeled after one of the crosslinking operations found in biology. The biological model that has probably attracted the most technological interest is the mussel glue that sets under moist conditions (Silverman H G, Roberto F F. Mar Biotechnol (NY) 2007, 9, 661-81. Deacon M P, Davis S S, Waite J H, Harding S E. Biochemistry 1998, 37, 14108-12). Cross-linking of the mussel glue is initiated by the enzymatic conversion of phenolic (i.e., dopa) residues of the adhesive protein into reactive quinone residues that can undergo subsequent inter-protein crosslinking reactions (Burzio L A, Waite J H. Biochemistry 2000, 39, 11147-53. McDowell L M, Burzio L A, Waite J H, Schaefer J J. Biol Chem 1999, 274, 20293-5). A second biological cross-linking operation that has served as a technological model is the transglutaminase-catalyzed reactions that occur during blood coagulation (Ehrbar M, Rizzi S C, Hlushchuk R, Djonov V, Zisch A H, Hubbell J A, Weber F E, Lutolf M P. Biomaterials 2007, 28, 3856-66). Biomimetic approaches for in situ gel formation have investigated the use of Factor XIIIa or other tissue transglutaminases (Sperinde J, Griffith L. Macromolecules 2000, 33, 5476-5480. Sanborn T J, Messersmith P B, Barron A E. Biomaterials 2002, 23, 2703-10).
One biomimetic approach for in situ gel formation of particular interest is the crosslinking of gelatin by a calcium independent microbial transglutaminase (mTG). mTG catalyzes an analogous crosslinking reaction as Factor XIIIa but the microbial enzyme requires neither thrombin nor calcium for activity. Initial studies with mTG were targeted to applications in the food industry (Babin H, Dickinson E. Food Hydrocolloids 2001, 15, 271-276. Motoki M, Seguro K. Trends in Food Science & Technology 1998, 9, 204-210), while later studies considered potential medical applications. Previous in vitro studies have shown that mTG can crosslink gelatin to form a gel within minutes, the gelatin-mTG adhesive can bond with moist or wet tissue, and the adhesive strength is comparable to, or better than, fibrin-based sealants (Chen T H, Payne G F, et al. Biomaterials 2003, 24, 2831-2841. McDermott M K, Payne G F, et al. Biomacromolecules 2004, 5, 1270-1279. Chen T, Payne G F, et al. J Biomed Mater Res B Appl Biomater 2006, 77, 416-22).